provider: Add multiple credentials and config files parameters

[gdavison]

The AWS SDK for Go v2 allows passing multiple shared credentials and shared config files. Note that the corresponding environment variables, AWS_SHARED_CREDENTIALS_FILE and AWS_CONFIG_FILE accept single files only.

1. Document support for both environment variables
2. Add parameter shared_credentials_files
3. Add parameter shared_config_files
4. Deprecate shared_credentials_file

[github-actions]

This functionality has been released in v4.0.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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